Superhydrophobic materials and surfaces that produce water contact angles in excess of 150° are being intensively studied in order to provide superior water repellency and self-cleaning behavior. This unique property is very useful in many industries, such as microfluidics, textiles, construction, automobiles, and so forth. Many examples of superhydrophobicity are found in nature, especially in plants and insects. For example, lotus leaves are superhydrophobic because of their rough-surface microstructure. Self-cleaning occurs as water droplets remove surface particles as they roll off the leaves. Superhydrophobicity also provides good buoyancy for floating on water. Another example from nature is the lady's mantle leaf that obtains its superhydrophobicity from a furlike coverage of bundled hairs. Interestingly, individual hairs are hydrophilic. However, the elastic deformation of the bundled hair ends away from the substrate results in a superhydrophobic surface. The bundling of the hairs is an example of the importance of curvature in hydrophobicity. This curvature effect is also very important in determining the oil-repellent (“oleophobic”) properties of the surface. Water strider feet and bird feathers are other famous examples of superhydrophobicity present in nature. By observing these features, one realizes that superhydrophobicity results from a combination of low surface energy and high surface roughness.
Several approaches have been reported for combining materials of low surface energy with high surface roughness. One approach is to roughen a normally smooth surface of a hydrophobic material. Plasma etching is widely used for this purpose. Mechanical stretching and microphase separation of fluorinated block copolymers have also been used. A second approach is to treat a rough surface with a hydrophobic material. Etching, lithography, and nanowires/nanotubes by chemical vapor deposition (CVD) have been used to produce a rough surface, followed by a hydrophobic coating to produce a low surface energy. Whereas these approaches are two-step processes, single-step approaches, such as sol-gel phase separation and plasma polymerization, can also produce a rough surface with low surface energy.
Electrospinning is a versatile technique for producing micro-nanofibers from many kinds of polymers. In a laboratory environment, electrospinning requires a high-power supply, a conducting substrate, and a syringe pump. The electro-spinning process is initiated by a high electric field between the syringe containing viscous polymer solution) and the conducting substrate. Because of the high electrical potential, a charged liquid jet is ejected from the tip of a distorted droplet, the so-called Taylor cone. This liquid jet experiences whipping and bending instabilities within a sufficient distance to evaporate its solvent thoroughly and, consequently, becomes a solid nonwoven micro/nanofiber membrane on the substrate. Oriented polymer nanofibers can also be produced by modifying the ground electrode geometry and/or rotating it and by using a microfluidic chip to deliver the solution to the ejection tip.
However, due to their relatively low dielectric constants, many hydrophobic materials are not susceptible to electrospinning Electrospinning has been used to make membranes with rough surfaces, followed by the deposition of hydrophobic material. For example, rough membranes are electrospun first and then coated with hydrophobic material by deposition techniques such as CVD and the layer-by-layer technique. However, this process can require additional cost and material to sufficiently coat the electrospun membrane.
Accordingly, a need exists for alternative methods for electrospinning hydrophobic coaxial fibers into superhydrophobic coaxial fiber mats.